1. Field of the Invention
The present invention relates to image forming apparatuses, and, in particular, to an image forming apparatus for forming images on a sheet, such as a copying apparatus, a laser-beam printer, an ink-jet printer, a facsimile, or a multifunction apparatus provided with a plurality of the functions of these apparatuses.
2. Description of the Related Art
The image forming apparatuses for forming images on sheets are, for example, a copying apparatus, a laser-beam printer, an ink-jet printer, and a facsimile. The copying apparatus generally reads images from an original and forms the images on a sheet in accordance with read image data. Some of the more recent models of copying apparatus are provided with a communication function for receiving image data from the outside and forming images on sheets in accordance with the image data. The laser-beam printer or the ink-jet printer generally forms images on a sheet in accordance with the image data sent from an external device such as a computer. The facsimile is generally provided with functions to read images from an original and for communication such as sending read image data to the outside and receiving image data from the outside so as to form images on a sheet according to the received data. In recent years, multifunction apparatuses each having a plurality of the functions of a copying apparatus, a printer, and a facsimile have been known.
Hitherto, the above image forming apparatuses including, for example, electrophotographic image forming apparatuses have been each provided upstream of a photosensitive drum with a pair of registration rollers which is rotatably supported by the lateral side plates of the image forming apparatus. The pair of registration rollers stops a sheet at a nip part thereof, the sheet being transferred from a sheet holder such as a cassette which holds the sheets, corrects the direction of transfer of the sheet (compensate for an oblique direction), and moves the sheet by controlling timing such that the sheet is positioned at an imaging position of the rotating photosensitive drum.
The roller pressure of the pair of registration rollers (a pressing force applied to each other) must be set high (approximately 2 kg or more) for compensation for an oblique direction of the sheet. The pair of registration rollers must be provided coaxially with clutches such as electromagnetic clutches for on-off-switching of a driving force transmitted from a drive source because the movement of the sheet must be suspended for a moment. Therefore, the pair of registration rollers is generally mounted on the lateral side plates which have strength sufficiently large for receiving the high roller pressure and supporting the heavy clutches.
Since the pair of registration rollers must be mounted firmly, a transfer path, instead of the pair of registration rollers, downstream of the pair of registration rollers generally opens when sheets jammed between the pair of registration rollers and a transfer portion for transferring a toner to the sheet are handled. In this case, the jammed sheets are taken out by being transferred with the pair of registration rollers being manually rotated.
However, when the transfer path opens at a position toward the transfer portion for handling a jam occurring in the known image forming apparatus in a state in which a sheet is firmly nipped by the pair of registration rollers at the trailing end of the sheet, there is a risk in that the leading end of the sheet comes into contact with a part, such as a guide, of the image forming apparatus. In this case, when the toner image has been formed on and has not been fixed to a leading-end part of the sheet disposed in the transfer portion, there is the risk that the toner image will adhere to the part, such as a guide, of the image forming apparatus. The toner image on the part such as a guide may adhere to and stain the sheet when image forming is again performed after the jam handling is completed.